In semiconductor applications, a charge pump is used to increase a small input voltage (for example, Vcc) into the larger voltages that are passed to the word lines and bit lines of semiconductor memory devices. These voltages affect the reading and writing of data from/to the memory device. The voltage is increased by channeling the relatively small input voltage through a series of stages. The more stages a charge pump has, the greater the resulting output voltage.
Rise time, or charge pump-up time (the amount of time necessary to convert the input voltage, Vcc, to the required output voltage) directly affects the time needed to read information from and write information to a particular byte of memory. Currently, charge pumps are constructed using several bootstrap capacitors having the same size (capacitance) C at each respective node of the charge pump. A bootstrap capacitor is defined as the capacitor connected to each respective node of a charge pump. Thus, to reduce rise time, the capacitance at each successive node is increased until the application defined speed requirement is met.
With conventional charge pumps, by increasing the capacitance at each stage the time required to generate the necessary output voltage decreases. However, the rise time does not improve significantly with the increase of capacitance. The decrease in rise time reaches a limit that cannot be surpassed. Thus, with conventional charge pumps, there is a limit to the attainable rise time.
As the required voltages for applications increases, the number of stages necessary to generate the higher output voltages also increases. Another drawback associated with conventional charge pumps is that with the increased number of stages, all having a common capacitance, the physical area taken up by the charge pump also increases.
A further drawback associated with conventional charge pumps is that as the number of stages increases, the power required to drive the charge pumps also increases due to the increased number of capacitors present in the charge pump.
Still a further drawback associated with conventional pumps is that as you increase the number of capacitors in a charge pump, the efficiency of the charge pump decreases.
Thus, there is a need for a charge pump having reduced rise time while at the same time taking up less physical area on an integrated circuit chip.